County of Marin Health and Human Services

Influenza Surveillance Update - Week ending December 13, 2014


During the first eleven weeks of the 2014-2015 influenza season (September 28-December 13), influenza activity in Marin remained minimal and within expected levels for early in the season.

Influenza-like illness activity: 

We updated the influenza-like illness (ILI) syndrome definition by broadening the list of chief complaints.  The weekly percentages of visits for ILI are higher throughout the year than the prior definition, but the same general patterns from prior years remain.  We applied the improved definition to all seasons in the graph below (Figure 1).

Influenza-like illness activity in Marin County was minimal during week 50 (December 7 - 13, 2014).  Discharge data from Kaiser San Rafael, Marin General Hospital, and Novato Community Hospital emergency departments and visits to Sutter Terra Linda Urgent Care showed the percentage of visits associated with influenza-like illness (ILI) was 8.61% during week 50.  See Figure 1 below.

Figure 1: Proportion of Visits due to Influenza-Like Illness (ILI):  Kaiser San Rafael, MGH, and NCH Emergency Departments and Sutter Terra Linda Urgent Care, 2012-13, 2013-2014, and 2014-2015 Influenza SeasonsInfluenza-like illness, Marin County, 2012-2015

PROVISIONAL DATA, SUBJECT TO CHANGE. The case definition for ILI is any chief complaint field that includes the following keywords or variants of the keywords: influenza, flu, influenza-like illness, pneumonia, fever, cough, and/or sore throat.

Influenza testing1:  During week 50, 3 (9.1%) of the 33 specimens tested for influenza were positive.  All positive specimens during week 50 were influenza A.  During this influenza season through December 13, 15 specimens have tested positive for influenza.

Influenza Tests in Marin County, 2014-2015

1. Laboratory results reported by Kaiser-Novato, Kaiser-San Rafael, Marin General Hospital, Novato Community Hospital, and Sutter Terra Linda Urgent Care.

Visit for influenza vaccination, testing, and treatment recommendations.


  • During week 50, the geographic distribution of influenza activity in California was regional*.
  • Outpatient ILI activity as a percentage of total visits to sentinel providers was 2.3% in week 50.
  • Of the 1,793 specimens tested by the Respiratory Laboratory Network and Sentinel Laboratories during week 50, 246 (13.7%) were positive for influenza; of these, 36 (14.6%) were positive for influenza B and 210 (85.4%) were positive for influenza A.  Of the influenza A specimens, 57 (27.1%) were subtyped as seasonal A (H3) and 153 (72.9%) were not subtyped.
  • Kaiser Permanente hospitalization data for northern and southern California showed that the percentage of hospitalizations for flu admissions, defined as a diagnosis of “flu,” “influenza,” or “pneumonia” recorded in text fields at time of admission, was 4.9% during week 50.
  • No specimens from California residents have been strain-typed this season.
  • California Department of Public Health has not received reports of influenza-associated deaths in persons under age 65 during the 2014-2015 influenza season to date.
  • The California Department of Public Health's Influenza website contains guidance and additional data.

United States

  • Influenza activity in the United States was increasing in week 50.
  • Some circulating H3N2 strains differ from the H3N2 component of this season's influenza vaccine.  The vaccine is still recommended.  CDC's indications for antiviral therapy are here.
  • Of the 20,064 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 50, 5,200 (25.9%) were positive for influenza.
  • The proportion of deaths due to pneumonia and influenza (P&I) was below the epidemic threshold.
  • Four influenza-associated pediatric deaths were reported during week 50.
  • The proportion of outpatient visits for influenza-like illness (ILI) was 3.7% during week 50.
  • Nine of 10 regions reported a proportion of outpatient visits for ILI at or above the region-specific baseline level during week 50.  ILI activity was high in 13 states, moderate in six states, low in New York City and five states, and minimal in 26 states during week 50.
  • During week 50, the geographic spread of influenza activity was reported as widespread+; in 29 states, regional* in 14 states, local^ in five states, and sporadic~ in two states.  See map below.
  • For additional national influenza data, visit the Center for Disease Control and Prevention's influenza surveillance website.

Geographic Distribution of Influenza in the US

+Widespread: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of the state with recent laboratory evidence of influenza in the state.
*Regional: Outbreaks of influenza or increases in ILI and recent laboratory confirmed influenza in at least two but less than half the regions of the state with recent laboratory evidence of influenza in those regions
^Local: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of the state.
~Sporadic: Small numbers of laboratory-confirmed influenza cases or a single laboratory-confirmed influenza outbreak has been reported, but there is no increase in cases of ILI.

Additional flu trends can be seen in Google Flu Trends which uses aggregated Google search data in a model created in collaboration with CDC to estimate influenza activity in the United States.  For more information and activity estimates from California, the U.S. and worldwide, see

If you have any questions or comments please contact:
Jessica Cunningham-Krahl, MPH
(415) 473-7346

This publication is provided to you by the Office of the Public Health Officer, County of Marin Department of Health and Human Services.


California Department of Public Health. Accessed 12/22/2014.

Centers for Disease Control and Prevention.  Accessed 12/23/2014.

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